The following configuration is known as a relay unit (relay circuit) that has the function of switching between current flow and no current flow to a load using contact relays.
That is, the relay unit includes a series circuit in which mechanical switches respectively provided in a plurality of contact relays are connected in series to each other. This series circuit is connected in series to a load and a load power supply. When all the mechanical switches are in the closed state, current flows through the load. On the other hand, when at least one of the mechanical switches is in the open state, the current flow through the load is interrupted. Examples of the relay unit are disclosed in European Patent No. 1202313 and German Patent No. 3541338.
The specifications of European Patent No. 1202313 (May 2, 2002) and German Patent No. 3541338 (May 27, 1987) are examples of background art.
When all but one mechanical switch are in the closed state and this one mechanical switch is switched from the open state to the closed state, current starts to flow through the load. On the other hand, when all the mechanical switches are in the closed state and one mechanical switch is switched from the closed state to the open state, the current flow through the load is interrupted. In both cases, at the moment at which the one mechanical switch is switched, an arc occurs from this mechanical switch, causing a damage to the contacts of the mechanical switch. Accordingly, the life of the contact relay including the mechanical switch in which the arc occurred will be shortened. As a result, there is the problem that the life of the relay unit is shortened.
This is explained more specifically in the following: Conventionally, it is common that all the mechanical switches of the series circuit are switched at the same time. On the other hand, in the series circuit, it is common that timings at which the mechanical switches are opened/closed vary from each other due to individual differences between the mechanical switches, the wear of the contacts of the mechanical switches (that is caused by, for example, on/off switching of the mechanical switches), or the like. As a result, the arc occurs intensively when a specific mechanical switch is switched, and the life of the contact relay including this specific mechanical switch is extremely shortened as compared with other contact relays.
Since the short life contact relay depends on the variation in the above-described timing at which the mechanical switch is opened/closed, it is difficult to specify this short life contact relay at the time of manufacturing of the relay unit. Therefore, in order to avoid the problem of the life of the relay unit, it is necessary to replace all the contact relays by non-contact relays or high arc-resistance relays. However, non-contact relays and high arc-resistance relays are more expensive than general-purpose contact relays, and thus it is difficult to realize an inexpensive relay unit when all the contact relays are replaced by non-contact relays or high arc-resistance relays.
The present invention was made in view of the above-described problem, and an objective thereof is to provide a relay unit that is inexpensive and has a long life, and a method for controlling a relay circuit.